Fix a few N+1 queries identified by Bullet

See !10263

app/controllers/admin/abuse_reports_controller.rb

 class Admin::AbuseReportsController < Admin::ApplicationController
   def index
     @abuse_reports = AbuseReport.order(id: :desc).page(params[:page])
+    @abuse_reports.includes(:reporter, :user)
   end
   def destroy

app/controllers/groups/group_members_controller.rb

@@ -14,6 +14,7 @@ class Groups::GroupMembersController < Groups::ApplicationController
     @members = @members.search(params[:search]) if params[:search].present?
     @members = @members.sort(@sort)
     @members = @members.page(params[:page]).per(50)
+    @members.includes(:user)
     @requesters = AccessRequestsFinder.new(@group).execute(current_user)

app/controllers/projects/merge_requests_controller.rb

@@ -39,6 +39,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @collection_type    = "MergeRequest"
     @merge_requests     = merge_requests_collection
     @merge_requests     = @merge_requests.page(params[:page])
+    @merge_requests     = @merge_requests.includes(merge_request_diff: :merge_request)
     @issuable_meta_data = issuable_meta_data(@merge_requests, @collection_type)
     if @merge_requests.out_of_range? && @merge_requests.total_pages != 0

app/controllers/projects/milestones_controller.rb

@@ -21,9 +21,9 @@ class Projects::MilestonesController < Projects::ApplicationController
     @sort = params[:sort] || 'due_date_asc'
     @milestones = @milestones.sort(@sort)
-    @milestones = @milestones.includes(:project)
     respond_to do |format|
       format.html do
+        @milestones = @milestones.includes(:project)
         @milestones = @milestones.page(params[:page])
       end
       format.json do